Grouping by PO_NUMBER is causing your problem. You want something more like the code below. You can change the order and format to suit your needs.

select
year(date_created) as Year,
datename(mm,date_created) as month,
count(po_number) as total_po
from
po_file
group by
year(date_created),
month(date_created),
datename(mm,date_created)
order by by
year(date_created),
month(date_created),
datename(mm,date_created)